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Monday, June 3, 2013

A new paper at PNAS by Botigué et al. takes a close look at African and Near Eastern admixture in Europe:

Human genetic diversity in southern Europe is higher than in other regions of the continent. This difference has been attributed to postglacial expansions, the demic diffusion of agriculture from the Near East, and gene flow from Africa. Using SNP data from 2,099 individuals in 43 populations, we show that estimates of recent shared ancestry between Europe and Africa are substantially increased when gene flow from North Africans, rather than Sub-Saharan Africans, is considered. The gradient of North African ancestry accounts for previous observations of low levels of sharing with Sub-Saharan Africa and is independent of recent gene flow from the Near East. The source of genetic diversity in southern Europe has important biomedical implications; we find that most disease risk alleles from genome-wide association studies follow expected patterns of divergence between Europe and North Africa, with the principal exception of multiple sclerosis.

The term "recent" is used throughout the paper to describe the IBD results, but as far as I can see there's no mention of any dates. Based on the data in the very thorough Ralph and Coop European IBD study (see here), I'd say that segments of over 1.5cM represent gene flow from well within the past 5,000 years. If this assumption is correct, then the results certainly make a lot of sense. That's because there were well documented historical events that could account for the main outcomes in the figure below: a) low level IBD sharing between Sub-Saharan Africa and much of Southern Europe; b) inflated IBD sharing between North Africa and Southwestern Europe; and c) inflated IBD sharing between Southeastern Europe and the Near East.

I probably don't need to discus in detail what these events might have been. Suffice it to say that the Mediterranean Basin has seen several major empires which facilitated regular population movements between Southern Europe, North Africa and the Near East. This process included the slave trade, which was one of the main economic activities in the region for a couple thousand years.

It's important to note, however, that fastIBD doesn't specify the direction of gene flow. In other words, shared IBD segments can be the result of our ancestors either receiving or giving admixture, or gene flow from a third party. But as Botigué et al. point out, the North African samples which show the highest IBD sharing with Iberians are also those with the lowest European ancestry proportions in the ADMIXTURE analysis (see below). Therefore, it's unlikely that this shared IBD is of European origin in any significant degree.

There's also a PCA in the supplementary PDF which further underlines that most of the IBD sharing between Europe and North Africa, as well as Qatar, is not of European origin, because it creates significant substructures within the European sample.

Unfortunately the Qataris are the only Near Eastern sample used in the study. Then again, if I was to pick a single ethnic group to represent the Near East in an IBD study like this, then Qataris would probably be near the top of the list. That's because they've been affected by population movements from other parts of the Arabian Peninsula and also Persia, but at the same time never experienced significant gene flow from Europe. More information about the genome-wide genetic ancestry of Qataris is available in this recent open-access paper by Omberg et al.

Botigué et al. also make some interesting comments about Jewish genetic ancestry in Europe. The quote below comes from the supplementary PDF.

Another possible hypothesis to explain the increased diversity in southern Europe is that an influx of Jewish ancestry had a heterogeneous effect on genetic diversity in Europe. However, in most European populations here, virtually no Jewish ancestry was detected. On average, 1% of Jewish ancestry is found in Tuscan HapMap population and Italian Swiss, as well as Greeks and Cypriots. This may reflect the higher sharing with Near Eastern populations in the Italian peninsula and southeastern Europe (Fig. 2C) or low levels of gene flow with the early Italian Jewish communities (6). Estimates from the IBD analysis are in agreement with ADMIXTURE estimates that the amount of sharing between these populations is extremely low (SI Appendix, Table S3). Specifically, results of IBD sharing between southwestern Europe and North Africa are two orders of magnitude greater than those found between the same region and Jews, the average WEA for southern Europe and North Africa is 203, while for southwestern Europe and European Jews is 1.3.